Approximately 80,000 American asthmatic women become pregnant each year. In the majority of these women, the course of their asthma changes during pregnancy, either for the better or for the worse. The mechanisms of these changes remain obscure, and ellucidation of these mechanisms should substantially improve the understanding and management of asthma and other related medical problems during pregnancy as well as aid in the understanding of the modulating factors involved in asthma in general, thus providing potential benefit to the nearly 9 million Americans with asthma. This proposal tests the hypothesis that, in comparison to women whose asthma worsens during pregnancy (W), women whose asthma improves during pregnancy (I) experience increased gestational glucocorticosteroid (GC) effects, associated with an increased amount of circulating cortisol, an increased number or affinity of GC receptors and/or a decreased amount of circulating competitors for GC receptor binding. The proposal also tests the hypothesis that this increased GC effect leads to increased beta-adrenergic responsiveness in I vs W women. Pregnant women with asthma and non-asthmatic controls will prospectively enter the study in their first trimester of pregnancy and be followed for the duration of pregnancy and for 6 months post-partum. Asthmatic women will keep symptom diaries, perform daily peak expiratory flow measurements and be evaluated medically with spirometry at least monthly during pregnancy and post-partum. Their course of asthma during pregnancy will then be compared subjectively and objectively to their course of asthma postpartum. Circulating leucocyte counts, anti-IgE induced basophil histamine release, GC receptor number and affinity, beta-adrenergic responsiveness, and plasma cortisol, progesterone, deoxycorticosterone and aldosterone will be serially assessed during pregnancy and post-partum. These parameters will then be compared in the following groups of women: (1) Documented I Women (2) Documented W Women (3) Women whose asthma did not change significantly during pregnancy compared to post-partum and (4) Non-asthmatic Controls.